As the quantity and speed of data communications over fiber optic systems rapidly increases due to the growing demand from Internet usage and other communications, improved all-optical switching systems are of increased interest to overcome the high cost and slow switching speeds of conventional switches. These conventional switches include, for example, various mechanical switches, electro-optic switches, and thermo-optic switches. In particular, the increased complexity and cost of switching systems that involve switching from an optical signal to an electrical signal and then back to an optical signal have increased the level of interest in improved all-optical switches.
An all-optical switch provides switching of an optical signal from one input path to a selected one of a plurality of different output paths without any intermediate conversion of the optical signal to an electrical signal. This is typically accomplished by applying an electrical signal to a switchable element to cause the optical signal to be selectively switched. These electro-optic switches are responsive to the electrical signal to selectively switch the light of the optical signal from the input path to the selected one of the output paths.
A variety of approaches are known for making all-optical optical switches, such as, for example, described in U.S. Pat. Nos. 6,687,431, 6,661,948, 6,625,346, and 6,549,699. The switchable element in many of these devices is a Micro-Electromechanical System (MEMS) optical element such as a MEMS mirror. The MEMS mirrors may be individually controllable and movable, e.g., pivotable (or rotatable) under analog (or continuous) control, such that, upon reflection, optical wavelengths are directed from one or more input ports into selected ones of the output ports. In this way, the optical switch may be used to selectively couple the wavelengths between the input and output ports of the system. However, a number of problems arise when such MEMS mirrors operating in an analog manner are used in an optical switch. For example, to avoid instability from mirror structure resonance a means for sensing the precise position of each MEMS mirror may need to be integrated with the MEMS device, which can be difficult and complex to achieve. In addition, switching individual MEMS elements into a MEMS device can cause disturbances in neighboring MEMS elements. In particular, switching MEMS mirrors in a MEMS array has been found to cause an aerodynamic coupling with other mirrors in the array that can disturb mirrors that are intended to remain static. The position of the MEMS mirrors can also drift because the mirrors are generally actuated by a relatively weak electrostatic force that requires a large voltage. The large voltage can cause charging that also disturbs the position of the mirrors. All of these effects become a more significant problem when these devices perform dynamic gain equalization by off-axis alignment of the free space beam as it is coupled into an output port. This technique requires in a much tighter tolerance in the mirror positioning because the attenuation sensitivity as a function of angular positioning level increases nonlinearly as off-axis alignment is extended to achieve higher attenuation levels. The higher sensitivity is the reason complex servo-loops and expensive mirror position sensing systems are disclosed as desirable for this technology.
Digital mirror arrays solve many of the problems with their analog counterparts, and a number of such arrays have become available which include an integrated monolithic switching structure. These devices use relatively low voltages to provide individual mirror elements that can be selectively switched between two or more positions with high precision and stability. One class of such digital mirror arrays are spatial light modulators, often referred to as digital micromirror displays or devices (DMDs). Typically, a DMD consists of an array of tiny mirrors (typically, several million per square inch), wherein the angular position of each mirror element is individually controllable between at least two positions that are angularly offset from one another by approximately 10 to 20 degrees, for instance. A mirror base is located behind the mirror elements. The individually addressable mirror elements are tiltably mounted on mechanical hinges, and typically the array of mirror elements overlays a layer of controlling circuitry in the mirror base, all of which is mounted on a semiconductor chip. The mirror face of the DMD is composed of a generally rectangular grid array of rectangular or square mirror elements. A typical mirror element is about 16 micrometers square, and the individual elements are separated from one another by a distance of about 1 micron. Individually controlled tilting of the mirror elements in the array around at least one axis allows energy that is reflected from the mirror face to be formed into a predetermined pattern. Further, the mirror face can be substantially instantaneously reconfigured in response to digital signals to form a different pattern. Such reconfiguration can generally be achieved in about 25 microseconds or less.
Optical switches incorporating DMDs are known and disclosed, for example, in U.S. Pat. Nos. 6,295,154, and 7,203,398 and U.S. Appl. Serial Nos. 2003/0164959 and 2002/0034356. These switches suffer from a number of limitations in extending the devices to higher levels of functionality to support multiple wavelengths or colors or light on each port, larger numbers of switching ports, or additional functionality such as independent control of the optical power level during switching
Accordingly, it would be advantageous to provide a multiple function DMD switching device that is capable of performing multiple optical processing functions In particular, it would be advantageous to provide a device that integrates a 1×N switch (where N>1) that can control the power levels directed to multiple output ports in a cross-talk free manner.